Exhaust Gas Turbocharger, Detailed Information - GF09.40-P-2500F
Engine176, 177
Shown on engine 177.9
Overview
This document contains information on:
- General
- Function
- Charge pressure control
- Overrun mode bypass air
General
The charging system of the engine model series M176 and M177 comprises two exhaust gas turbochargers connected in parallel (Biturbo) in the hot "interior V" of the 8-cylinder V engine. Exhaust manifolds and exhaust gas turbochargers are specially insulated due to their location.
Function
Charge pressure control
The boost pressure is controlled electropneumatically via the boost pressure control pressure transducer. The vacuum is generated by the mechanical vacuum pump attached to the engine. To control the boost pressure, the pressure transducer is actuated by the engine management control unit according to a performance map and according to load. To do this, the engine management control unit evaluates the following sensor signals and functions of engine management:
- Knock control, transmission overload protection (overheating protection).
- Left charge air temperature sensor and right charge air temperature sensor (charge air temperature).
- Pressure sensors downstream of air filter, left and right cylinder bank (intake air pressure).
- Pressure sensors upstream of throttle valve on the left and right (boost pressure).
- Pressure sensors downstream of left and right throttle valve (engine load).
- Accelerator pedal sensor (load request made by the driver).
- Crankshaft Hall sensor (engine speed).
In wide open throttle operation, maximum boost pressure builds up.
To reduce the boost pressure, the exhaust flows that drive the ATL are each redirected through bypasses by opening the boost pressure control flaps. To do this the boost pressure control pressure transducer actuates the boost pressure control flap vacuum units with vacuum from the vacuum pump. The vacuum units react by closing the boost pressure control flap linkages over a rod, which close the bypasses.
If there is no vacuum at the vacuum cells, the boost pressure control flaps and thus also the bypasses are opened. The boost pressure control flaps therefore allow the exhaust flow to bypass the turbine wheels (bypass), thus controlling the boost pressure and limiting the turbine speed. In this way the boost pressure can be adapted to the current load demand on the engine.
To monitor the current boost pressure, the pressure sensors upstream of the left and right throttle valve send the corresponding voltage signal to the engine management control unit.
The charge air temperature is detected in the charge air coolers by the left and right charge air temperature sensors and sent to the engine management control unit in the form of a voltage signal.
Overrun mode bypass air
The ATL continues turning for a period of time after the start of deceleration mode due to the inertia of the shaft, compressor and turbine wheel. In the case of rapid closing of the throttle valve, a charge pressure wave therefore runs back to the compressor impeller. This charge pressure wave would create a condition with a low delivery volume and high pressure conditions at the compressor impeller, which causes charger pumping (brief howling and mechanical stress). Opening the bypass air switchover valve prevents this through rapid depressurization through a bypass in the intake side of the ATL.